1. Field of the Invention
The present invention relates to the improvement of a sensorless motor driving circuit.
2. Description of Related Art
Sensorless motor driving circuits for driving, for example, two-phase brushless motors are well known. These circuits for driving two-phase brushless motors use rotation detecting elements such as Hall elements and switch over the conduction of the driving current (conduction) in the excitation coils utilizing an induced voltage (counter-electromotive voltage) generated in the excitation coils.
These general sensorless motor driving circuits carry out conduction switching by detecting the induction voltage in the excitation coils and supplying a certain amount of delay with respect to the timing of the inversion of the polarity.
Spike voltages (fly-back voltages) generated at the time of conduction changeover are then removed with filters.
Further, if the motor rotor is not activated directly after conduction in the excitation coils, with the motor rotor already being in the stationary position (referred to as the reference position) that is intended, if an induction voltage for the excitation coils is not detected within a certain period of time, an activation pulse is generated and the conduction pattern is forcibly switched over.
These methods of providing a prescribed delay in relation to the timing with which the polarity inverts, providing filters for removing the spike voltages, and generating activation pulses, can be divided into analog methods and digital methods.
Circuits for the analog methods utilize CR time constants to put in phase delays, remove spike voltages and generate activation pulses. Circuits for the digital methods, on the other hand, almost all use microprocessors.
Use of the digital method is possible in systems where the circuit scale is large but is not viable in small-scale circuit applications because of the cost of the microprocessors. It is therefore more natural for the analog method to be adopted, rather than the digital method.
However, in the analog method, it is necessary to set up the time constants for each element of the CR time constant circuit in the most appropriate manner, but this proves to be difficult due to interference between the time constants for each element. Further, a large number of resistors and capacitors are necessary, which means that a large number of parts are required.